1. Field of the Invention
This invention relates to dicarboxylic acid mono-(2-hydroxydodecyl)-esters, to salts thereof and the use thereof as corrosion inhibitors in aqueous systems.
2. Discussion of Related Art
Water-carrying plants, such as a steam-generating plant, heating systems, cooling water circuits and closed waterline systems are always exposed to the corrosive effect of the water circulating in them which is directed primarily against the base metals of the particular systems, for example steel, brass, aluminum, zinc or galvanized steel. The risk of corrosion is further increased by the fact that high temperatures generally prevail in such plants and the circulating water contains constituents which also chemically promote the corrosive attack on the particular materials. Accordingly, chemicals which are intended to guard against or completely prevent corrosion have long been added as corrosion inhibitors to the water circulating in the afore-mentioned systems. In this connection, particularly good results have been obtained with phosphorus-containing compounds, for example phosphonic acids or inorganic phosphates, which are optionally combined with zinc salts. Hitherto, the effectiveness of combinations such as these has been entirely satisfactory.
However, the recently discovered relationship between a high phosphate content of natural waters and eutrophication, which has even resulted in legal stipulations relating to the constituents of raw water systems of water-carrying plants, has led to the requirement that such raw waters be substantially or completely free from phosphorus-containing compounds. In addition, the phosphonic acids or inorganic phosphates mentioned, as corrosion-inhibiting additives to raw water, have the further disadvantage from a practical and technical point of view that they also promote increased biological growth within the cooling systems so that microbicides must also be added to the systems to inhibit such growth.
Since relatively hard waters are also occasionally used in raw-water circulation systems of the type herein, the use of phosphate-containing corrosion inhibitors additionally leads to the formation of apatite or apatite-like deposits which, like the known boiler scale, considerably complicate the transfer of heat and hence lead very quickly to operational disturbances. In addition, such deposits are extremely difficult to remove, particularly in closed circuit systems.
Problems also arise where zinc salts are used in combination with phosphorus-containing corrosion inhibitors of the type under consideration. Zinc salts are generally known to be very toxic to fish so that the waters of the type herein must not under any circumstances enter the effluent. In addition, the self-purifying power of natural waters is distinctly inhibited at zinc concentrations of only 0.1 ppm and higher. Further, the use of combinations of zinc salts with phosphonic acids or phosphates generally leads at relatively high pH values (pH&gt;8.0) to increased siltation of the raw-water circulation system through the precipitation of zinc hydroxide.
The corrosion-inhibiting effect of dicarboxylic acid semi-amides and derivatives thereof, particularly succinic acid semi-amides, is known from German Patent Application No. 33 00 874. Unfortunately, the disadvantage of using those compounds is their inadequate solubility in water.
Thus, an object of the present invention is to provide substantially phosphorus- and zinc-free corrosion inhibitors for aqueous systems which are effective at only low concentrations, are easy to produce and which premanently or completely prevent the corrosion of numerous materials of the type used in plant construction. The compounds used should be harmless in their behavior with respect to the environment and should satisfy the legal requirements imposed in that connection, particularly in regard to their toxicity.